If the receiver says Magnum and the serial number ends in 'M', it is a Magnum and it good for anything 3" or shorter. BUT, as Dfariswheel said, if it has a 2-3/4" barrel on it, you should not shoot anything longer than 2-3/4" shells in it with that barrel. The barrels are completely interchangeable, but the standard receivers were supposed to have a 2-3/4" ejector and ejector spring set up. Other than the ejector and spring, and the rollmarking, the receivers are identical.